Monte Carlo simulation of proton irradiation into a patient has been performed using CT data obtained for real proton treatment. We examined practical capability of this simulation method in treatment planning system in proton therapy in a point of calculation time, a model of secondary particle effects in proton irradiation, and possibility in use of dose distribution function produced with the Monte Carlo simulation. The GEANT is applied to the simulation in the proton therapy. It is a particle-based full simulation code including nuclear interaction. A patient structure is constructed with voxels, whose properties are determined by CT data. In the proton therapy, the simulation requires an atomic number Z and an atomic mass A of the material in each voxel. Thus an electron density in the CT data should be converted into Z and A in the proton simulation. Effective atomic number Z_<eff> and atomic mass A_<eff> are defined as : Z_<eff>=Σω_iZ_i, A_<eff>=Σω_iA_i, ω_i=P_iA_i/ΣP_iA_i, wher
… Moree P_i is a number weight of the i'th component in a material. They are expressed as a function of the CT data using calibration curves determined from phantom data. Each 5mm-thick slice of the CT data has 320×320 pixels in 1mm×1mm size. The patient structure is constructed as a group of voxels, as setup in the simulation code as planned in the treatment planning system. We developed the model with the following three points in the Monte Carlo simulation : (1) calculating energy deposit and kinematical parameter values once for a proton passing through each voxel, (2) making tables of spatial and kinematical distribution of only protons for each ridge filter and each proton energy in production at the front of compensator (bolus filter), and (3) using extended-depth-dose distribution in estimating energy deposits of primary and secondary protons passing through voxels. In introducing this model to the Monte Carlo simulation with the GEANT code, calculation time is 1-2 hours in producing dose distribution in a patient body under the following calculation condition : one PC workstation (Linux OS, Pentium4), 1% accuracy for each voxel cross-section of 1 slice (5mm×2mm), in a circle of 7cm in radius perpendicular to the proton beam at 250MeV. Independence of each event calculation in the Monte Carlo simulation makes it possible to reduce computing time 1/n with using n parallel sets of PC workstation. Clusterization of these PC workstation sets makes statistics n times easily in summing up events. We consider that this method is capable as treatment planning system in proton therapy, with several parallel sets of PC workstation. Less